This application claims the priority benefit of Taiwan application serial no. 89117744, filed Aug. 31, 2000.
1. Field of Invention
The present invention relates to a semiconductor manufacturing process. More particularly, the present invention relates to a method of manufacturing a dual damascene structure.
2. Description of Related Art
As semiconductor manufacturing reaches the deep sub-micron stage, dual damascene processing and associated techniques such as copper-filling and chemical-mechanical polishing becomes indispensable.
To cram large number of metallic interconnects on a limited chip surface, a multiples of metallic layer are often formed on a silicon chip. Earlier method of manufacturing a multiple of metallic layers includes the steps of providing a substrate having a first metal line layer thereon, forming an inter-metallic dielectric layer over the substrate, forming a via opening that exposes a portion of the first metal line in the inter-metal dielectric layer, depositing conductive material into the via opening to form a via plug, depositing a second metallic layer over the inter-metallic dielectric layer, and carrying out conventional photolithographic and etching processes to pattern the metallic layer into a second metal line layer. However, some defects may result from metal layer patterning process because of the difficulties in controlling dry etching. Consequently, a dual damascene processing method is invented. In dual damascene processing, an inter-metallic dielectric layer is formed over a substrate having a metal line thereon. Since an inter-metal dielectric layer has a higher etching rate than a metallic layer, a via opening that exposes the metal line and a shallow trench for forming upper layer metal line can be easily formed in the dielectric layer. A liner layer is formed over the exposed surface of the dual damascene opening. Metallic material is deposited into the dual damascene opening so that a via plug and an upper metal line are formed at the same time. Excess metal outside the dual damascene opening is removed by polishing using a metal-reactive solution to expose liner layer is exposed. Finally, the liner layer is removed by polishing using a liner-material-reactive solution to form a dual damascene structure.
However, once the metal layer above the liner layer is removed in a conventional chemical-mechanical polishing process, a portion of the metal inside the dual damascene structure will be removed as well. This is because liner material has a lower polishing rate than metal when the metallic layer is polished with a metal-reactive reagent. Hence, the upper metal line generally has a dishing surface that result from over-polishing.
FIGS. 1 through 3 are cross-sectional views showing the progression of steps for forming a dual damascene structure in a conventional dual damascene process.
As shown in FIG. 1, a substrate 100 is provided. A metal line 102 is formed above the substrate 100 and then an insulation layer 104 such as a silicon dioxide layer is formed over the substrate 100.
As shown in FIG. 2, photolithographic and etching processes are carried out in sequent to form a dual damascene opening 105 in the insulation layer 104. A liner layer 106 such as a titanium/titanium nitride layer is deposited over the exposed surface of the dual damascene opening 105. Copper is deposited over the substrate 100 and filled the opening 105 to form a copper layer 108.
As shown in FIG. 3, excess copper outside the dual damascene opening 105 is removed by polishing using a copper-reactive solution to expose the liner layer 106. Finally, the liner layer 106 outside the dual damascene structure is removed by polishing using a liner-material-reactive solution.
After a conventional chemical-mechanical polishing operation, a dish surface may form on the upper portion of a metal line. Dishing may be more serious for metal lines having a somewhat wider line width. Dishing occurs because liner material has a lower polishing rate than metallic material. Hence, metal on the dual damascene structure is removed faster than the liner layer on each side leading to over-polishing.
Because a conventional dual damascene process tends to lower thickness of metal lines through dishing, cross-sectional area of a metal line is reduced and hence corresponding resistance for a piece of the metal line is increased.
Accordingly, one object of the present invention is to provide a method of fabricating a dual damascene structure capable of preventing the dishing of metal lines.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of fabricating a dual damascene structure. A substrate is provided and then an insulation layer is formed over the substrate. A dual damascene opening is formed in the insulation layer. A liner layer is formed over the exposed surface of the dual damascene opening. Metallic material is deposited over the substrate filling the dual damascene opening to form a metallic layer. A cap layer is formed over the metallic layer. Due to the presence of the dual damascene opening, the cap layer above the dual damascene structure is at a level lower than surrounding region. Cap layer outside the dual damascene opening region is removed by polishing using a cap-material-reactive solution. A portion of the metallic layer is removed by polishing using a metal-reactive solution until the cap layer above the dual damascene structure is exposed. The polishing is continued to remove metal outside the dual damascene structure until the liner layer is exposed. Finally, the liner layer and the cap layer are removed.
According to the embodiment of this invention, the polishing rate of cap layer material must be smaller than the metal material when the metallic layer is polished using a metal-reactive solution. Hence, the metal in the dual damascene structure is protected from over-polishing, thereby preventing dishing of metal lines.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.